Typical hard disk drive systems record information on circular disks, each disk having a multiplicity of tracks concentrically located thereon. Each disk drive normally contains a plurality of disks, each disk recording surface having one or more magnetic heads which transfer information to or from an external system. Each magnetic head is located on an arm, and all arms are aligned vertically and attached to a common head positioner assembly. The head positioner assembly is driven by a motor so that the arms and magnetic heads move uniformly across the surfaces of the vertically aligned disks. Head positioner assemblies are usually mounted to rotate the arms and magnetic heads along an arcuate path over the disks.
In normal operation, a host controller issues a track seek command. The controller electronics respond to the track seek command, determining the direction and magnitude of movement necessary to drive the heads from their current position to the destination position. Hard disks are generally divided into a plurality of concentric tracks, and data is read or written along each track. Further, position of the head on each disk, position of data on each disk, and the desired destination for arm movement is indicated by track indices. High performance disk drive units typically attempt to maximize the density at which tracks are written. In order to achieve higher densities, the positioner motor is energized, and the data read/write head counts the number of tracks it crosses until it reaches the destination. For example, if the head is at track 25, and the electronics require movement to track 256, the data head must read 231 track crossings. The drive motor accelerates to a maximum value for a calculated period of time and decelerates so that it attains zero velocity on reaching the destination.
Typically, the carriage on which the heads are mounted is incorporated in a servo system, which performs the necessary positioning functions. Three different types of servo systems exist: a dedicated servo system, which has servo information located on one of many disk surfaces, an embedded servo system, wherein each data surface is used to store the servo information, and the hybrid servo system, which uses a dedicated disk surface and a low sample rate embedded servo on each data surface to store servo information. The servo system senses the position of the heads by means of servo signals recorded in tracks on the disk surfaces. Embedded servo and hybrid servo system accuracy and data storage has been the object of developments within the disk drive field, as the accuracy at which the head can be made to follow the track centerline determines the density of the tracks located on a disk. The closer the head can be made to follow the track centerline, the closer together the tracks can be spaced on the disk, and storage efficiency increases.
A typical hard disk used in an embedded servo or hybrid servo system has a plurality of outwardly radiating "spokes" representing servo data. Each set of sectors between "spokes" represents zones where data is stored. Servo signals are typically arranged in frames, including an AGC field, sync mark, cylinder information, and servo burst information. Typically, four radially extending servo burst groups, designated A, B, C, and D, are used to assist in positioning the heads, as the signals generated by the heads in reading these four burst groups determine the radial position of the head.
Track centerline positions are determined by the system by comparing the servo burst signals received. That is, knowing the placement of servo burst data on the disk, the position of a head traversing a track can be calculated from the signals received.
Previous systems employing multiple servo burst groups used the following equation to determine position information: ##EQU1##
Alternately, various calculations using C and D in the denominator have been used, or constant value scale factors, to determine head position. The drawback of these systems was the requirement to calibrate for demodulator offsets, demodulator gains and head width variations, as well as the inherent nonlinearities associated with ratios of sums and differences. Demodulator gains include any imprecision in head positioning due to various gains applied to the sum and difference ratios used to determine position information. Gain rounding or inaccuracies adversely affected head positioning. With respect to demodulator offset, signals received must be demodulated to recover position information and convert it into the appropriate position signal for use by the position servo. This demodulation and conversion tends to create dc offsets which may be introduced into the position signal. With respect to head variations, strength of the signal generated in the servo transducer is a function of the actual gap width of the servo transducer, which affect the magnitudes of the servo burst signals (A, B, C, or D) received, introducing error into calculations which use these servo burst signals. Further, prior compensation schemes were inherently nonlinear, introducing unwanted position movement into the system.
Accordingly, it is an object of the current invention to provide an embedded or hybrid servo system having accurate head positioning capability.
It is another object of the current invention to compensate for demodulator offsets and head width variations in positioning data read/write heads. Signals received must be demodulated to recover position information and convert it into the appropriate position signal for use by the position servo. This demodulation and conversion tends to create dc offsets which may be introduced into the position signal. With respect to head variations, strength of the signal generated in the servo transducer is a function of the actual gap width of the servo transducer, which affects the magnitudes of the servo burst signals (A, B, C, or D) received. Thus, head variations must be minimized to avoid introducing error into calculations which use these servo burst signals.
It is still another object of the current invention to provide a linear head positioning system which does not cause unwanted position movement.